Method for recording seismic waves



Uw 8, W42- J. P. MINToN 2,304,740

METHOD FOR RECORDING SEISMIC WAVES Filed March 6, 1940 2 Sheets-Sheet l TMIE f Ja 'Q5 fw' DEQ., 8, 1942. J, p, MlNTQN 2,304,740

METHOD FOR RECORDING SEISMIC WAVES Filed March 6, 1940 2 Sheets-Sheet 2 Q, l: Q, k Q: Q g gg k Q h Q ulgu Cl---Ill atented Dec., 8, 1942 Ares t.

traint IWTHO'D FR. RECORIDDIG SEISMIC WAVES .lohn P. Minton, Dallas, Tex., assigner, by mesne l assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application March 6, 1940, Serial No. 322,503 1 claim. (ci. rit- 352) to distinguish from the record of these waves the exact instant at which the first energy in aparticular reflection; arrives at the detecting instrument. This is primarily because the amplitude of the reflection is very slow in reaching its maximum. Therefore, in order to be able to distinguish the instant of arrival of the first energy in aparticular reflection it is necessary that some means be provided for building up the wave front of the initial portion of the reflection and still retain the characteristics of the reflection wave form by whichv it has been distinguished heretofore.

The subject matter of the present application is the provision of a method and means for accomplishing the result recited above. In addition `to detecting the seismic waves, amplifying them and recording them in the conventional manner, means are provided whereby derivatives of the wave passing through the conventional amplier can be taken electrically and combined with the function. The resultant signals will not only have, when recorded, the conventional energy envelope but will have in addition thereto an initial portion of abrupt wave front that will enable the "observer not only to distinguish the reflections tional manner and combined with a selected crl der of derivative of the electrical signals.

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Still another object of this invention is the tional amplier to produce a composite signa that may be recorded as the function of a reilected wave plus one or more of its derivatives of selected order.

This invention further contemplates a method and apparatus for recording all of the detected energy from a single reflected wave at substantially constant amplitude.

Other objects and advantages will become apparent from the following detailed description when viewed in the light of the drawings, in

' which Figure 1 is a diagrammatic illustration of the Vreflection seismograph operation showing the shot point, two geophones, amplifiers and recording galvanometer;

Figure 2 is a group of curves illustrating a reflected wave, the same wave after having 1, 2 and 3 derivatives taken of it, and the combination of the reflected waves and the derivatives;

Figure 3 is a detail circuit diagram of a conventional amplifier connected in parallel with a derivating amplifier; V

Figure 4 is a diagrammatic illustration of an arrangement of amplifiers and derivating stages showing an arrangement whereby the wave function and a selected order of derivatives can be combined to give a resultant wave which can be recorded 0n the recording galvanometer; and

Figure 5 is a modification of the arrangement shown in Figure 4.

Referring to the drawings in detail, particularly Figure 1, there is illustrated a section of the earths surface showing the different strata which form interfaces from which seismic waves are t reflected. Seismic waves are created at the shot point S by the detonation of an explosive charge. These waves travel outwardly from the shot in all directions and comprise a complex wave train of many frequencies. Some travel directly through the surface of the earth to the geophones G1 and G2 along the path a. Others travel downwardly and strike a more dense strata along which they are refracted and finally reach the geophones Where they are detected, While others. those in which we are primarily interested, travel downwardly from the shot point to the interfaces of the substrata from'which they are reflected along the paths b to the geophones where they are detected.

The geophones that are used for the detection s of these seismic waves can be of any conven tional type but preferably of the type which will generate an electrical signal that will correspond in frequency and amplitude to the seismic waves striking the geophone. These generated electrical signals are conducted to amplifiers where they are amplified and recorded on a pluasoma@ A rality of traces by a recording galvanometer of conventional design.

As illustrated by the curve fut) in Figure 2, the ilrst energy in a particular redaction is quite weak and as a result in many cases it is imposemphasizing this first part.` of thereflection;

Since, in interpreting seismograms, an'attempt ,l

is made to ready the instant ofarrival ofy a re-` ection to /loooseconds. it' becomes quiteimportant that the first energy to reachy the geo` phone should be suilciently emphasized by rthe apparatus that a definite break will occurin the trace when recorded. This problem. has been' solved by the present invention'by detecting and amplifying the reflection in the usualway but in' addition, taking a selected numbergof derivatives of the wave toemphasize the build up. time-and adding these derivatives back to the function. and then recording the composite results of the function and the derivatives. l The result will be as shown in Figure 2, a reflection which still retains its distinguishingfeatures but -having a wave front that is sumciently abrupt that,an interpreter can readily distinguishthe instant at which the first energy was detected bythe geophone. f In Figure 2 f (:v). f(:v) and f"() illusformer Zl'aareimpi'essed onv the grid circuit ZtlV "of an" amplifier tube 2l. lThe output signalsA *from the-.tubellgappearing in the plate circuit 22- andreslstor 23 'produce signal voltages acrossv the resistor, infamanneryvell-known `to those familiar,withresistance coupled amplifiers, that can.beimpressedacross thederivator V2li thatI comprises( the' condenser `2li and .resistance 23 vconnected in series. If the impedance'lof the f condenserfli isy made vquitehigh relative to that nected nirilparallel with the amplier 'described above. Conductors I3 and' I3 place the pri- .mariea of the' input transformers T1 and Ta in 'parallel relationship. Therefore, the same signals thatare introduced to therst amplifier *will lbe introduced to the second or derivating amplifier.-` I' The signals fromthe secondary ofthe trans` 'of.the."resistor 28,:then. a'volt'ager can, be' tapped ofjof the,res`istor`26 vby the conductor '27 and f I the` ground circuit that will be substantiallythe first-derivativeof the volta'gegoccurring across s Duev to thev high va trate lrst, second and third :order of derivatives l When these derivatives function, theresult will of the function fw); are combined with the be as shown at curves, taking the derivative ofi the f uncti'on'f.;v

'fier tube'333 whereit is further amplified'andl conducted to the: 'recording'. galvanometer 'by produces attenuation to such a degreethat the i distinguishing feature of the function is not disturbed when these derivatives are added ,to it.

In Figure 3 there is shown a detailed circuit diagram by means of which detectedl seismic waves may be amplified and recorded in the manner taught by this application. wThe `seismic waves are detected by the geophone G and the` i electric signals that are generated vinthe geophone that correspond to the seismic waves are conducted by the conductors Il) and Il to the primary of a transformer T1. The transformer T1 is the input transformer of a conventional resistance coupled amplifier which utilizes pentode tubes l2 and I3 in the first two stages and a triode i4 in the last stage. The outputtransformer T2 of this amplifier is connected in the input Y circuit of a recording galvanometer R by means of the conductors I5 and I6. i l

There is shown associated with the recording galvancmeter conventional timing means l1 that are adapted to cooperate with the recording galvanometer in marking definite indications of the passage of time on the seismogram while recording the seismic waves.

If the amplifier described above was used alone` 1 for amplifying the seismic Waves,I the record obtained would be conventional in form. That is, all reections would be very weak at first, making it very hard or almost impossible to distinguish when the first energy arrived. at the detecting instrument. To overcome this objectionable feature, a derivating amplifier `is con` 7 a wave function plus Ythe secondl derivative ofl having the desired characteristics. 65

`the resistance 23.

voltage appearlng',across the resistance 23, ls

j4 introduced in the grid circuit, of amplifier tube-23' where Iit is vamplified in, a manner wellfknovvn 'in'. the' art. -Th e voltageappearing across the g resistor 29 as aresultV of the flow of current in y the' plate circuit of tube ZB'isagainderivated byl I 'means of the V second derivative taking stage l3l) which comprises the. condenser'l andthe re- A I sistance 32." Thefgresultant voltage acros'sthe' resistor 32Wi111novv be substantially a derivative of a' derivative or the second derivative of the inputgvoltage vto i the amplifier. The voltage corresponding to the second derivative may then be v impressed on the grid circuit' of a thirdxamplimeans of th'e. transformer-T4," the. conductors 34,' /35,|5and|.. j Since the primary windings of'transformers duce Aa composite record. that will be made up of the function. 'I'he resulting recorded. 'reflection in the trace will have the distinguishing features by which it has been recognized heretofore and in addition, Will have its first Weak energy portiongemphasized suiiicientl'y that the, interpreter of. therecord can instantlyselect the exact in- 'stantv at vvhich the rstenergy'fof the reflectionconventional. amplier channel to, give results rangement is illustrated in Figure' 4. In this arrangement, the input vterminals ofl they conventional amplifier 36 arev connected in parallel With'thein'put terminals oi'defrivatlng amplifiers 31; 33 and- 39 by Vmeans of the conductors Ml and 3|. The" outputsignalsfrom these `amplifiers are respectively` impressed across the resistances ,32, 33, M and 45. These resistancesare all connected in vseries and in the input circuit of the recording galvanometer R by leads 46 yandflll.

ttenuaftion .efiected 'by de- 'ri 'v ating, it is. often necessary to interpose amplii iication' stages `between derivative taking stages.` ,Therefore, the, voltage across'the resistancel 26 Y. v`-fwhich issub'stantially'th'e first derivative of the Another arto derivating amplier 6l.

generated by the geophone without distortion. Amplifier 31 will take the rst derivative of the signal voltage from the geophone, amplier 38 the second derivative and amplifier 39 the nth derivative. By means of switches 48, anyone or any combination of the derivative taking ampliers can be connected or disconnected from the circuit in such a manner that the operator can record the function wave alone, or the function Wave plus any order of derivative which he may desire. Each of the amplifiers 31, 38 and 39, in addition to having derivative taking stages such as described in connection with Figure 3 may be provided with any desired number of amplification stages. This particular arrangement has sufficient flexibility that any combination of ampliers can be selected by the operator by the mere manipulation of the switches 48. i

Still another arrangement of the apparatus is illustrated diagrammatically in Figure 5. Waves which are detected by the geophone G are rst amplified in the conventional manner by the amplifier 49. The amplified signals from the output of the amplifier are impressed across the resistance 50 by means of leads 5I and 52 and the input of derivating ampliiier 53. The output signals from the derivating amplifier 53 are impressed across the resistor 54 by means of the leads 55 and 55 and the input to amplifier 51. The output signal from amplier 53 iWill be the first derivative of the voltage impressed across the resistance l). The output from derivating amplifier 51 is impressed across the resistance 58 by means of leads 59 and 60 and across the input The output signal voltage from the derivating amplifier 51 will therefore be a derivative of a derivative or a second derivative of the amplied function that is impressed across the resistance 50. The output of the amplier 6l is connected across the resistor 62 by means of the leads 63 and 64 and across the input of another derivating amplifier B5. The signal across the resistor 62 will be substantially the third derivative of the function. The signal voltage output from the derivating amplifier 65 is impressed across the resistor 66 by means of the leads 61 and 68. The resistances 50, 54, 58, 62 and 65 are all connected in series and by means of the leads 69 and 10 connected in the input circuit of the recording galvanometer, in such a manner that the voltages across these resistances will add to produce a record of the waves having the desired characteristics. By means of switches 1|, like the arrangement disclosed in Figure 4, any combination of the amplifiers can be used to produce a record having the desired characteristics.

Although'the subject matter of this invention has been described in connection with a single geophone, it is obvious to those skilled in the art that it is applicable to an electric seismograph that employs a plurality of geophones, ampliers and traces.

The detailed circuits of the various elements used in the process and apparatus above have not been specically described inasmuch as they are commonly known and may be varied within relatively wide ranges without departing from the principles of this invention.

I ciaim: A A method of recording reflected seismic Waves that comprises the steps of detecting the seismic waves by generating corresponding electrical signals, amplifying a portion of the generated signals directly, taking a selected order of derivatives of at least one other portion of the generated signals, adding the derivatives singly or in combination to the amplified first portion such that the resultant signals will be at least in part in phase with the portion of the directly amplified signals and recording the sum on a single trace.

JOHN P. MINTON. 

